Question

6. What is the difference between the MPN test and the MF test? 7. How is KF Strep agar selective and differential for fecal strep bacteria?

Answer 1

Answer:

6):-

• MPN technigue only indicates presence or absence of approximate number or organisms by observing turbidity in test tubes. And using MF technique discrete colonies of bacteria are isolated.
• Using MF technique 100ml of water sample can be passed through membrane and organisms grows on membrane and then discrete bacterial colonies are transferred to confirmation media. But in MPN technique only 2ml sample in pour plate technique or 0.1ml sample on spread plate technique can be analyzed.
• For chlorinated water effluent MF technique gives most accurate results than MPN technique.
• Coliforms can be detected within 16 to 24 hours after molecular filter when grown on Endo broth by MF technique. MPN technique is time consuming than MF technique

7):-

• KF strep agar contains triphenyl-tetrazolium chloride stain which makes it effective for the selective isolation and enumeration of fecal Streptococci and is specifically recommended for food and water testings.
• It contains lactose and maltose which is the carbohydrate source for most of the fecal streptococci.
• It also contains sodium azide which is the selective agent that suppresses the growth of the gram-negative bacterium.
• The acid formed during fermentation/metabolism is detected by bromocresol purple that indicates a color change from purple to yellow.
• Then the added triphenyl-tetrazolium chloride stain that is absorbed during the process of metabolism by the bacterial cells results in the change of color of colonies from pink to red.
• So basically, in KF strep agar, the sodium azide is the selective agent which promotes the growth of fecal Streptococci by suppressing other gram-negative bacteria.
• And two dyes bromocresol purple and triphenyl-tetrazolium chloride which changes the color or agar medium and colonies respectively making the medium differential for fecal Streptococcus bacteria.

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